70 SCIENCE 
are local maxima of over 50 inches on the 
slopes well exposed to moist winds; but 
minima of less than 40 inches in the valleys. 
The extremes are over 80 inches on the ex- 
posed southern face of the Appalachians where 
North Carolina, South Carolina and Georgia 
meet; and under 30 inches in the enclosed 
Champlain valley. Without the abundant 
moisture which sweeps northward unobstruc- 
ted all the way from the Gulf of Mexico, the 
Great Lakes could hardly exist. Since they 
are present, they exert a local effect on the 
climate; and increase the rainfall by perhaps 
5 inches, making the total thereabouts 35 
inches. Contrasts between windward and lee- 
ward shore rainfalls are not marked, for the 
light precipitation which occurs frequently 
with the cool westerly winds, and the heavy 
rainfall which comes with the less frequent 
easterly winds nearly balance. Of the well- 
watered eastern half of the United States, Pro- 
fessor W. M. Davis says: 
The world hardly contains so large an area as 
this so well adapted to civilized oceupation.3 
West of the 95th meridian, the rainfall 
lines run north and south instead of east and 
west, aS is the case to the east. At about the 
100th meridian the rainfall becomes too small 
for ordinary methods of farming, being less 
than 20 inches in the north and under 25 
inches in the south. From here west to the 
Rockies the rainfall decreases almost to ten 
inches; so the Great Plains region is one of 
grazing, dry farming, or local irrigation. In 
the outlying highlands and the mountain 
front, the rainfall again rises to 15 or 20 
inches. In comparison with the heavily for- 
ested east this open country was easily—in 
some areas, too easily—settled; but the fluc- 
tuations of rainfall in this marginal region 
make man’s hold too precarious to favor a 
dense population. 
The Interior Plateau and Basin region, 
walled off by high mountains, is arid. The 
rainfall of the northern Rockies exceeds 40 
inches in Idaho, but is under 80 inches else- 
3‘¢Hlementary Meteorology,’’ Boston, 1894, p. 
301. 
[N. 8S. Vou. XLVIIT. No. 1229 
where; the central Rockies locally enjoy more 
than 30 inches, but the high plateaus of the 
south receive but 15 to 25 inches. The lower 
mountains and plateaus and the valleys in the 
rain-shadow of the Cascades and Sierras are 
arid, with less than 10 inches of rainfall. 
This aridity becomes extreme in the south; 
there, with lesser cyclonic activity, and 
greater heat, the rainfall averages under 5 
inches a year. Water for the irrigation of 
these driest regions is not altogether lacking, 
for, except in the south, they occur in the lee 
of the wettest mountains. Thus, the Cascades 
with rainfall 10-15 times as great as that in 
the Yakima valley, supply abundant water for 
this great orchard. 
The cause of aridity in the rain-shadow of 
the Cascades and Sierra Navadas is apparent 
from a glance at the excessive rainfall on the 
western side of the coast ranges and these 
higher mountains. South to the 40th parallel 
the rainfall exceeds 80 inches, and on the west 
flank of the Olympics, even 120 inches. In 
California, the rainfall decreases rapidly south- 
ward, while on the mountains of southern Cal- 
ifornia, the amounts are under 30 inches, and 
on the coast at San Diego even less than 10. 
The cause of the heavy rainfall is the rapid 
cooling of the moist air which is blowing al- 
most continuously from the Pacific. This 
cooling is brought about (1) by the expansion 
of the air as it is forced to rise over the ob- 
structing mountains; (2) by the similar cool- 
ing as this air rises in the numerous cyclones; 
and (8) by cooling to the cold ground in 
winter. Diminishing cyclonic activity and in- 
creasing warmth of the land cause a south- 
ward tapering of the rainfall. The trough be- 
tween the coast ranges and the higher moun- 
tains on the east receive only half as much 
rainfall as the mountains on either side; thus 
in many parts of the valleys irrigation is nec- 
essary particularly in the San Joaquin valley 
and in southern California. Water is sup- 
plied abundantly by the slow-melting moun- 
tain snows. Unlike the eastern United States, 
then, the western United States has sharp 
contrasts of rainfall in short distances; and 
because the rainfall is excessive on the moun- 
